The present invention relates to a device that reduces a peak factor (a ratio of maximum instantaneous power to average power) of a transmission signal.
In orthogonal frequency division multiplexing (OFDM) employed in WiMAX, 3GPP LTE, or the like, peaks having large power appear in a transmission signal. Since a power amplifier of a wireless transmitter has nonlinearity, distortion occurs in an output signal if a transmission signal including peaks is input. This distortion deteriorates transmission characteristics and becomes the causes of spurious radiation.
Thus, a device having excellent input-output linearity is used in a power amplifier to decrease the distortion of a transmission signal including large peaks. Moreover, the peaks included in a transmission signal are reduced to increase the output power level without saturating the power amplifier.
A peak factor reduction device disclosed in JP 2012-19401 A is known as a conventional art of this technical field.
JP 2012-19401 A discloses a peak factor reduction method in which, when a peak cancellation signal Y=X(|X|−A0)/|X| equal to a portion of a multi-carrier signal X exceeding a threshold A0 is created, first, signals y1Lo1=X|x1|(|X|−A0)/|X|/(|x|+|x2|+ . . . ), y2Lo2=X|x2|(|X|−A0)/|X|/(|x1|+|x2|+ . . . ), . . . , and the like are created, and the peak cancellation signal Y is created as the sum of these signals such that Y=y1Lo1+y2Lo2+ . . . =X(|X|−A0)/|X|. A peak reduced output OUT=X−Y=A0X/|X| is obtained, and the amplitude is limited to A0.